1. Field of Invention
This invention is generally directed to imaging members for electrophotography. More specifically, this invention is directed to a process for preparing a charge generator layer for electrophotographic imaging members, and to electrophotographic imaging members produced thereby.
2. Description of Related Art
In electrophotography, an electrophotographic substrate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface. The plate is then exposed to a pattern of activating electromagnetic radiation, such as light. The light or other electromagnetic radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This image developing can be repeated as many times as necessary with reusable photoconductive insulating layers.
An electrophotographic imaging member may take one of many different forms. For example, layered photoresponsive imaging members are known in the art. U.S. Pat. No. 4,265,990, which is incorporated herein by reference in its entirety, describes a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. Thus, in photoreceptors of this type, the photogenerating material generates electrons and holes when subjected to light.
More advanced photoconductive photoreceptors contain highly specialized component layers. For example, a multilayered photoreceptor that can be employed in electrophotographic imaging systems can include one or more of a substrate, an undercoating layer, an optional hole or charge blocking layer, a charge generating layer (including photogenerating material in a binder) over the undercoating and/or blocking layer, and a charge transport layer (including charge transport material in a binder). Additional layers such as an overcoating layer or layers can also be included.
The photogenerating layer utilized in multilayered photoreceptors typically include, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive material may be formed as a continuous, homogeneous photogenerating layer.
In photoreceptors of the above type, the photogenerating material generates electrons and holes when subjected to light. In the case of a photoreceptor including a hole blocking layer, the blocking layer prevents holes in the conductive ground plane from passing into the generator from which they would be conducted to the photoreceptor surface, thus erasing any latent image formed thereon. The hole blocking layer does permit electrons generated in the generator to pass to the conductive ground plane, preventing an undesirably high electric field to build up across the generator upon cycling the photoreceptor.
Certain layered imaging members are known, including those comprised of separate charge generating layers, and charge transport layers and overcoated photo responsive materials containing a hole injecting layer overcoated with a hole transfer layer, followed by an overcoating of a photo generating layer; and a top coating of an insulating organic resin. Such imaging member designs are described, for example, in U.S. Pat. Nos. 4,265,990 and 4,251,612, the disclosures of which are totally incorporated herein by reference. Examples of photo generating layers disclosed in these patents include trigonal selenium and phthalocyanines, while examples of transport layers include certain aryl diamines as mentioned therein.
Additional references illustrating layered organic electrophotographic photo conductor elements with azo, bisazo, and related compounds include U.S. Pat. Nos. 4,390,611, 4,551,404, 4,400,455, 4,390,608, 4,327,168, 4,299,896, 4,314,015, 4,486,522, 4,486,519, 4,555,667, 4,440,845, 4,486,800, 4,309,611, 4,418,133, 4,293,628, 4,427,753, 4,495,264, 4,359,513, 3,898,084, 4,830,944 and 4,820,602, the disclosures of which are totally incorporated herein by reference.
One conventional charge generating layer binder widely used in the art is PCz, a z-form polycarbonate produced by Mitsubishi Chemical Corporation, and having the following structure: ##STR2##
PCz is a Bisphenol-z type polycarbonate. These polycarbonates were chosen because they had the necessary properties for photoreceptor use, and also because they were soluble in non-halogenated solvents.
U.S. Reissue Pat. No. Re 33,724, the entire disclosure of which is incorporated herein by reference, discloses "z" polycarbonates containing an unsubstituted or substituted carbon ring. These polycarbonates are useful as binder materials for forming charge generating or charge transport layers of a photoreceptor.
U.S. Pat. No. 5,554,473, which is totally incorporated herein by reference, discloses a charge transport layer binder that is stated to provide wear resistance. The binder used in this patent requires a Tg of less than 200.degree. C., since a higher temperature causes stress cracks on the charge transport layer surface.
Despite these various known designs for photoreceptors, a need continues to exist in the art for photoreceptor designs that provide high quality products at lower cost. For example, although the z polycarbonates provide acceptable results for photoreceptor materials, they are comparatively expensive, thus increasing the production cost for the photoreceptor.
Furthermore, z polycarbonates introduce important constraints into photoreceptor design. For example, a wide range of molecular weights of the z polycarbonates is not generally available, thus making it more difficult to adjust such manufacturing steps as the coating process. Generally, such z polycarbonates are available only in molecular weights in the range of about 15,000 to 60,000. Furthermore, many of the z polycarbonates are not highly soluble in non-halogenated solvents. As a result, halogenated solvents must be utilized in the coating process. Such halogenated solvents, however, are becoming increasingly less desirable from the standpoint of environmental and safety concerns.
Thus, a need continues to exist in the art for improved materials that can be used as binder materials for photoreceptors. A need also exists in the art for means to reduce the manufacturing cost and environmental concerns of the manufacturing process, while still providing high quality products.